As is generally known in the art, such industrial vehicles as forklift trucks, hydraulic excavators and telescopic handlers make use of pressurized fluid to perform an intended task. The pressurized fluid is produced by means of a reversible hydraulic pump which in turn is drivingly connected to an engine. The power of the engine is also utilized to have the industrial vehicles go forwards and backwards at different speeds selected by the operator.
In order to enable the operator to change the speed and the direction of movement of the industrial vehicles, a variety of power-shifted transmissions have been employed, including: parallel shaft type transmissions; and planetary gear type transmissions. Most of the state-of-the-art parallel shaft type transmissions are so constructed as to establish three forward gear ratios and three reverse gear ratios depending on the operator's desire. These type of transmissions tend to require an increased number of gear shafts, bearings and hydraulically operated clutches, which may of course makes the transmissions structurally complicated, bulky and heavy weight. Another drawback of the conventional parallel shaft type transmissions is that the number of gears meshed and the number of clutches engaged for establishing an intended gear ratio would be necessarily increased, thus deteriorating operability and durability of the transmissions.
As a solution to the afore-mentioned shortcomings, use has been made of the planetary gear type power-shifted transmissions which is usually provided with a sun gear, planetary pinion gears, a ring gear, hydraulically operated clutches for causing these gears to be coupled together for rotation as a unit and hydraulically operated brakes for releasably securing these gears to a transmission casing against any rotation. Two or more forward gear ratios can be established by selectively actuating the clutches and the brakes through the use of pressurized fluid. Although the planetary gear type transmissions have compact structure as compared with the parallel shaft type transmissions, they are costly to fabricate and hard to diversify gear ratios, thus heavily restricting the freedom of design of the transmissions.